Cannot set horizontal margin on my blog

How do I set the horizontal margin for my blog? On the October 1 posting it has a horizontal scroll bar; is there a way to get rid of it and just show the whole entry on 1 screen? Should I pick a different theme?
Blog url: http://gihfblog.wordpress.com/

Posts > All Posts, change all your latest posts to Draft, click Update, change them back to Published, click Update again. (You can edit all 3 of them at once: tick the square in front of their titles then select Edit from the Bulk Actions dropdown and click Apply.)